These plates are of close grained
cast iron. They are amply ribbed to minimize deflection
under load. The composition of the metal, the
depth and cross-section of the ribs and the thickness
of the plate are designed to conform to the deflection
limits specified in IS 2285 -1963.
Surface plates are manufactured
to two tolerance standards. - Grade-I and Grade-II.
Grade-I plates are more precise than Grade-II
plates. The deviation in flatness for Grade-I
and Grade -II plates for sizes upto and including
2000 x 1000 mm are as prescribed in IS 2285-1963.
Grade-1 plates are artificially
seasoned.They are hand scraped to achieve maximum
local deviation from flatness of 5 microns over
any length of 300 mm at any points on the working
face , in plates of 630 X 400 mm and larger.
Aspecial scraping technique is used to achieve
a uniform distribution of load bearing points
and oil pockets. An average of 15-20 load bearing
points , after finish scraping , is standard.
This finish scraping technique ensures that
the area of the load bearing points increases
after the intial dulling of the high points.
This guarantees a long working life, at the
required accuracies.
Grade-II plates are accurately
machined to get the requisite flatness.
